You're equating the final stage of this approach to the entire approach. The point of this technique is that you get the benefits you typically would from dirty

You're equating the final stage of this approach to the entire approach. The point of this technique is that you get the benefits you typically would from dirty rectangles without the burden of the bookkeeping you would traditionally have. Using this technique your application "redraws" everything as if it's drawing it fresh each frame and the renderer cache takes care of determining what's actually changed. Typically with dirty rectangles you would have to manage this state in the application code, for example, determining that line X was edited then updating the region for that line, or determining that view Y moved and updating a dirty rectangle based upon it's previous and current positions.